1. Field of the Invention
The present invention relates to a display board illuminating device for a display unit including a display board, such as a dial scale of a tuner or a dial plate.
2. Description of Background Information
FIG. 1 shows an example of conventional devices for illuminating a display board. As shown, this conventional device includes a light guide plate 2 placed to face the back of an elongated rectangular display board 1 and a light emitting element 3, functioning as a light source, disposed to face an end surface of the light guide plate 2 at one side in the longitudinal direction of the light guide plate 2. The back surface 2a of the light guide plate 2 is formed into a rough surface or covered with a white printing material in order to diffuse the light.
In the illuminating device having the construction described above, the light emitted from the light emitting element 3 enters the light guide plate 2, and guided to a predetermined area of the back surface of the display board 1, while being reflected and diffused within the light guide plate 2.
In the case of the conventional illuminating device as described above, there was a problem that the light from the light emitting element 3 does not reach sufficiently to the opposite end of the display board especially when the size of the display board in the longitudinal direction is large, thereby making it difficult to illuminate the whole area of the display board 1 at a uniform illumination level. Furthermore, the light issued from the light emitting element 3 is not efficiently introduced to the back surface of the display board 1, causing a problem that the illuminating light is dissipated wastefully.